The proposed research is aimed at determining how the neural mechanisms controlling daily rhythms differ in nocturnal and diurnal mammals. A small group of neurons in the mammalian brain called the suprachiasmatic nucleus (SCN) is responsible for the generation of 24 hour rhythms. To date, most research into the SCN and how it controls circadian rhythms has been done with nocturnal rodents. Until recently there has been no suitable diurnal rodent model with which to investigate these issues. These investigators have been working with a diurnal rodent, Arvicanthis niloticus, that they recently imported to the United States from East Africa. In the research proposed here they intend to use these rodents to determine how the neural structures controlling circadian rhythms differ in nocturnal and diurnal animals. The first objective is to compare nocturnal and diurnal animals with respect to rhythms in the concentration of a molecule that regulates gene expression, within the SCN and several brain regions to which it projects. Second, the investigators will compare rhythms in metabolic activity of the SCN in nocturnal and diurnal animals. Third, they will use SCN transplants to determine whether activity rhythms of diurnal and nocturnal animals differ because of differences within the SCN or differences in responsiveness to signals emanating from the SCN. Finally, they will evaluate rhythms within one brain region that receives signals from the SCN and controls rhythms in hormone secretion. It is important to understand these issues because the circadian system influences virtually every physiological and behavioral variable, and this influence is profoundly different in nocturnal and diurnal species.